UPDATE: I realized i had an older [June 2017] rpi3-u-boot.bin in the boot partition. Need to update that - doh - expecting it will cure the issue.

Subject says it. I'm trying to boot a known-working (on an RPi3 B) Fedora 26 image on my new RPi3 B+ (yes, I'm using an official RPi 2.5A power supply) but only get the rainbow screen and the four slow / four fast LED blinks. According to this sticky:

viewtopic.php?t=58151
"If you have one of the latest PI 3B+ which won't boot and it keeps stuck on the rainbow screen, while also displaying the under voltage (lightening bolt) icon, then you are simply are using boot code that is too old. Please download the very latest version of Raspbian, or do an upgrade to it....Other OS's users might have to wait for their OS to get an upgrade."

So...is Fedora 26 in that category, and I just need to wait? Or has anyone found the secret sauce to get it to boot? Or do I just need to give up and move to Raspbian?